1. Field of the Invention
The invention relates to the logging of subsurface formations traversed by a borehole, by means of logging sondes displaced along the borehole and equipped with a sensor-carrying pad urged into contact with the borehole wall. More specifically, the invention relates to a logging technique adapted for use in stressed boreholes.
2. Description of the Prior Art
Tectonic stress is known to cause borehole breakout along the direction of least horizontal stress. In that case, the borehole has a larger dimension ("long axis") in the direction of breakout than in the perpendicular direction ("short axis"). A borehole with breakout is exemplified on FIG. 1, which is an ultrasonic image of a 20 centimeters high section of the borehole, shown as if seen from above. This example shows that the shape of the borehole cross-section may be quite irregular, and far indeed from a pure geometrical figure such as an ellipsis or an oval. For that reason, such boreholes shall simply be referred to hereinafter as "non-circular".
When logging sondes having a wall contact pad, such as those producing density and microresistivity logs, are run in a non-circular borehole, the pad and the sonde with its backup spring or spring-loaded arm which urges the pad into contact with the borehole tend to align with the "long axis" (breakout axis). Unfortunately, as illustrated on FIG. 1, the "long axis" regions of the borehole wall are rugose as a result of breakout, which results in a poor contact between the pad and the borehole wall, thus severely affecting the quality of the logs.
A technique known as "short-axis logging"has been implemented as an attempt to obtain good quality logs in non-circular boreholes. The purpose is to ensure contact of the pad with the borehole wall in its "short axis" regions, which, as shown on FIG. 1, are smoother and more susceptible to provide good contact with the pad than the "long axis regions". In order to align the pad with the "short axis", the sonde is equipped with extra springs, arranged so that the potential energy of the total spring system is minimized when the sonde is aligned along the "short axis". Such arrangement has led to some improvement over short sections of the borehole, but does not reliably align the sonde with the "short axis": in nearly circular boreholes, proper alignment is not ensured, and in sections with breakout, it was found that the sonde remained aligned with the long axis despite repeated attempts to correct its orientation.